α,β-unsaturated acids, particularly acrylic acid and methacrylic acid, and the ester derivatives thereof are useful organic compounds in the chemical industry. These acids and esters are known to readily polymerize or co-polymerize to form homopolymers or copolymers. Often the polymerized acids are useful in applications such as superabsorbents, dispersants, flocculants, and thickeners. The polymerized ester derivatives are used in coatings (including latex paints), textiles, adhesives, plastics, fibers, and synthetic resins.
Because acrylic acid and its esters have long been valued commercially, many methods of production have been developed. One exemplary acrylic acid ester production process involves the reaction of acetylene with water and carbon monoxide. Another conventional process involves the reaction of ketene (often obtained by the pyrolysis of acetone or acetic acid) with formaldehyde. These processes have become obsolete for economic, environmental, or other reasons.
Another acrylic acid production method utilizes the condensation of formaldehyde and acetic acid and/or carboxylic acid esters. This reaction is often conducted over a catalyst. For example, condensation catalyst consisting of mixed oxides of vanadium, titanium, and/or phosphorus were investigated and described in M. Ai, J. Catal., 107, 201 (1987); M. Ai, J. Catal., 124, 293 (1990); M. Ai, Appl. Catal., 36, 221 (1988); and M. Ai, Shokubai, 29, 522 (1987). These catalysts have a vanadium:titanium:phosphorus molar ratio of 1:2:x, where x is varied from 4.0 to 7.0, and have traditionally shown that the catalyst activity decreases steadily as the phosphorus content increased. The highest selectivity with respect to the aldol condensation products, e.g., acrylic acid and methyl acrylate, was obtained where x was 6.0. With these catalysts, the molar ratio of vanadium to titanium was maintained at or below 1:2. The acetic acid conversions achieved using these catalysts, however, leave room for improvement.
Even in view of these references, the need exists for improved processes for producing acrylic acid, and for an improved catalyst capable of providing high acetic acid conversions and acrylate product yields.